Change Session Values field to be non-exported

[dghubble]

• Add Session methods to access session state key/value pairs
• Use Session Set to set a key/value pair
• Use Session Get to get a value for a given key
• Use Session GetOk to get a value for a given key and whether the key exists in the map

// add user-id to session
- session.Values["user-id"] = 123
+ session.Set("user-id", 123)

// get user-id from session
- userID := session.Values("user-id")
+ userID := session.Get("user-id")